1. Field of the Invention
The present invention relates to a piezoelectric sound transducer which uses a piezoelectric plastic material. In particular, the present invention relates to a sound transducer which is essentially developed from a substrate layer and a layer of a piezoelectric plastic material deposited thereon, the piezoelectric plastic layer covering the substrate layer not in its entirety, but having openings.
2. Description of the Related Art
Sound transducers for converting electrical signals into acoustical signals and vice versa in various forms have been known in many technology fields for more than a hundred years. Different principles of electro-acoustic conversion are known. In addition to electromagnetic transducers as they are used in coil loudspeakers, for example, and electrodynamic or electrostatic transducers, as employed in microphones or headphones, for instance, piezoelectric sound transducers of the type used in ultrasonic sensors, for example, are known as well. Apart from the use of piezo ceramics, the use of piezoelectric plastics for converting electrical signals into acoustical signals and vice versa is known in the field of piezoelectric sound transducers. One known plastic material having piezoelectric properties is polyvinylidene fluoride (PVDF). The piezoelectric properties of PVDF form the basis of various sound transducer designs. Vapor deposition or sputtering methods, for example, are employed to deposit an electrically conducting surface on a PVDF foil. By applying an electrical field, the obtained structures may be used as sound emitters, or when using a charge transfer, as sound receivers. Another transducer principle is the piezo-composite principle, in which hard piezo materials, in particular, are combined with each other.
U.S. Pat. No. 4,638,207 describes the use of a piezoelectric polymer based on polyvinylidene fluoride (PVDF) to produce balloon-shaped loudspeakers. Toward this end, strips, which are made up of a PVDF polymer layer enclosed between an outer and an inner coating, are applied on a balloon, or the balloon itself is formed by a PVDF polymer layer enclosed between an outer and an inner coating.
In the described sound transducers on the basis of piezoelectric plastic materials, movement of the plastic layer occurs both in the layer plane, referred to as transverse expansion, and in the thickness direction of the layer, which is referred to as longitudinal expansion. However, the transverse expansion of the layer is locally impeded by the surrounding plastic material, which impediment occurs both in the transmitting operation, i.e., when using the sound transducer as loudspeaker, and in the receiving operation i.e., when using the sound transducer as microphone. This hindrance in the transverse expansion results in restricted dynamics of the sound transducer. In particular the deflection of the plastic layer out of the plane, which is required to generate or absorb a sound wave, is markedly restricted.